1. Field of the Invention
The present invention generally relates to semiconductor devices having transistors such as those called insulated gate type transistors, metal insulator semiconductor (MIS) type field effect transistors and MOS transistors. The present invention specifically relates to a semiconductor device suitable for being mounted on a liquid jet apparatus such as an ink jet apparatus, a DNA chip and an organic transistor and also relates to a liquid jet apparatus using such a semiconductor device. The liquid jet apparatus is used as an output terminal of a copying machine, a facsimile, a word processor, a computer and the like.
2. Related Background Art
An example of a semiconductor device used with a liquid jet apparatus will be described.
A semiconductor device for a liquid jet head has electrothermal conversion elements, switching elements for switching the electrothermal conversion elements, and a circuit for driving the switching elements, all being fabricated on the same substrate.
FIG. 20 is a schematic cross sectional view showing a portion of a liquid jet head having a conventional structure.
Reference numeral 901 represents a semiconductor substrate made of single crystal silicon. Reference numeral 912 represents a p-type well region, 908 represents an n-type drain region having a high impurity concentration, 916 represents an n-type drain region having a low impurity concentration, 907 represents an n-type source region having a high impurity concentration, and 914 represents a gate electrode, these elements constituting a switching element 930 made of a MIS type field effect transistor. Reference numeral 917 represents a silicon oxide layer serving as a heat accumulation layer and an insulating layer, 918 represents a tantalum nitride film serving as a heat generating resistor layer, 919 represents an aluminum alloy layer serving as a wiring layer, and 920 represents a silicon nitride layer serving as a protective layer, these elements constituting a main base 940 for a recording head. An area 950 is a heat generating area, and ink is jetted out of a jet port 960. A top plate 970 together with the main base 940 defines a liquid path 980,
Large current is required to drive a large load such as an electrothermal conversion element. If a conventional MIS type field effect transistor 930 is used for driving the electrothermal conversion element, the pn junction between the drain and well region cannot resist a high electric field generated by a reverse bias. Leak current is therefore generated and the breakdown voltage necessary for a switching element is hard to be satisfied. In addition, if an on-resistance of the MIS type field effect transistor used as the switching element is large, current is wastefully used so that current necessary for driving the electrothermal conversion element becomes hard to be obtained.
In order to solve the problem of the breakdown voltage of the switching element, a double diffusion metal oxide semiconductor (DMOS) 20 shown in FIG. 21 may be used.
In FIG. 21, reference numeral 152 represents a main base on which an electrothermal conversion element 141 serving as a load, a DMOS transistor 20 and a MOS transistor (not shown) are integrated. Reference numeral 153 represents a jet port, 154 represents a wiring electrode, 155 represents a liquid path, and 156 represents a top plate.
The structure of the DMOS transistor 20 is different from that of a general MOS transistor. A channel is formed later in the drain so that the drain can be made deep and its impurity concentration can be made low. The problem of the drain breakdown voltage can therefore be solved.
Although the DMOS transistor 20 has the characteristics sufficient for a high breakdown switching element, it is not versatile.
The reason for this will be specifically described with reference to FIG. 22. FIG. 22 is a circuit diagram of a circuit which has a load and a switching element and flows current through the load by controlling the operation of the switching element.
With the circuit arrangement shown in FIG. 22, if the power supply voltage VDD is set to 5.0 V or 3.3 V, a high level voltage signal output from an AND gate 46 is VDD. This signal is applied to a CMOS circuit 52 such as a CMOS inverter and input to the gate control electrode of the switching element 41.
An important point is the value of a voltage VHT applied to the CMOS circuit 52. The voltage VHT determines a voltage to be applied to the gate of the switching element 41. The value of VHT is required to be designed so that the on-resistance of the switching element 41 becomes lowest. If the on-resistance is made lowest, the size of a MOS transistor constituting the switching element, i.e., the area of an integrated circuit chip occupied by the MOS transistor, can be made smallest.
If this voltage is to be generated in the one chip integration circuit including the circuit shown in FIG. 22, it is necessary to change the voltage level from the power supply voltage VH to the voltage VHT in the integration circuit.
A transistor source follower circuit is used as the circuit for changing the voltage level, i.e., as a level shift circuit. The constant voltage VHT may be obtained by using the transistor source follower circuit (refer to Japanese Patent Application Laid-Open No. 10-034898, U.S. Pat. No. 6,302,504).
According to the knowing of the present inventor, no problem occurred if a sufficient drain breakdown voltage of a source follower transistor of the level shift circuit is obtained under the conditions of a highest power supply voltage VH of 30 V, a lowest reference voltage VGNDH of 0 V and a middle reference voltage VHT of 12 V.
However, the source of the source follower transistor was broken down when the highest power supply voltage VH was raised to 33 V, the lowest reference voltage VGNDH was set to 0 V and the middle reference voltage VHT was raised to 15 V, because of the reverse bias voltage of 15 V applied across the pn junction between the source and well regions of the source follower transistor.
Although attention is generally paid to the drain breakdown voltage, according to the knowing of the present inventor, attention is also required to be paid to the source breakdown voltage if the circuit shown in FIG. 22 is used at high power supply voltages.
An object of the invention is to provide a semiconductor device having a high source breakdown voltage, high performance and high reliability, and a liquid jet apparatus using such a semiconductor device.
Another object of the invention is to provide a semiconductor device capable of stably flowing large current through a load and capable of high integration, and a liquid jet apparatus using such a semiconductor device.
According to a main aspect of the invention, there is provided a semiconductor device having a switching element for flowing current through a load and a circuit for driving the switching element, respectively formed on a same substrate, wherein: the circuit comprises a source follower transistor for generating a drive voltage to be applied to a control electrode of the switching element; and a source region of the source follower transistor comprises: a first doped region connected to a source electrode; and a second doped region having an impurity concentration lower than an impurity concentration of the first doped region, the second doped region forming a pn junction with a semiconductor region forming a channel.
In the semiconductor device of the invention, it is preferable that the switching element is a DMOS transistor, the DMOS transistor comprising: a low impurity concentration drain region made of semiconductor of a second conductivity type and formed in a principal surface of a semiconductor substrate of a first conductivity type; a semiconductor region of the first conductivity type formed in the low impurity concentration drain region; a gate electrode as the control electrode formed via an insulating film on a surface where the pn junction between the semiconductor region and the low impurity concentration drain region; a source region of the second conductivity type formed on one end side of the gate electrode; and a drain region of the second conductivity type formed in the low impurity concentration drain region and having an impurity concentration higher than an impurity concentration of the low impurity concentration drain region; and the source follower transistor is an insulated gate type transistor having characteristics different from the DMOS transistor.
It is preferable that the drain of the source follower transistor comprises a first doped region connected to a drain electrode and a second doped region having an impurity concentration lower than an impurity concentration of the first doped region, the second doped region forming a pn junction with the semiconductor region forming the channel.
It is preferable that the drive voltage generated by the source follower transistor is applied to the control electrode via a CMOS circuit.
It is preferable that the second doped region of the source follower transistor is shallower than a depth of the low impurity concentration drain region.
It is also preferable that the second doped region of the source follower transistor has a depth same as a depth of the low impurity concentration drain region.
In the invention, it is preferable that the semiconductor region is formed deeper than the low impurity concentration drain region.
A plurality of DMOS transistors is disposed in an array without involving a dedicated element separation region.
It is preferable that the circuit comprises a low voltage CMOS circuit and a high voltage CMOS circuit to be controlled by the low voltage CMOS circuit, and a MOS transistor of the first conductivity type constituting the high voltage CMOS circuit is a DMOS transistor.
It is also preferable that a MOS transistor of the second conductivity type constituting the high voltage CMOS circuit comprises a low impurity concentration drain region and a high impurity concentration drain region having an impurity concentration higher than an impurity concentration of the first low impurity concentration region, respectively formed in a well region of the second conductivity type.
In the invention, it is preferable that the circuit comprises: a low voltage CMOS circuit; a high voltage CMOS circuit to be controlled by the low voltage CMOS circuit, the high voltage CMOS circuit applying the drive voltage to the control electrode of the switching element; and a voltage converting circuit for receiving a low voltage signal from the low voltage CMOS circuit and outputting a high voltage signal to the high voltage CMOS circuit.
It is preferable that the voltage converting circuit comprises a CMOS inverter and a MOS switch, the MOS switch being connected to a source of one MOS transistor constituting the CMOS inverter and turned on and off synchronously with the one MOS transistor.
Alternatively, the voltage converting circuit may include a CMOS inverter having a MOS transistor of one conductivity type with a plurality of gate electrodes to which a same phase signal is applied.
In the invention, an electrothermal conversion body serving as the load is integrated with and connected to a drain of the switching element.
The characteristics are at least one selected from a group consisting of a threshold value, a breakdown voltage, and a substrate current.
According to another aspect of the present invention, there is provided a liquid jet apparatus for jetting out liquid by utilizing heat generated by an electrothermal conversion body, comprising: a semiconductor device described above; and a jet port provided in correspondence with the electrothermal conversion body serving as the load.
The liquid jet apparatus further comprises a container for accommodating liquid to be supplied to the electrothermal conversion body.
According to still another aspect of the invention, there is provided a liquid jet apparatus for jetting out liquid by utilizing heat generated by an electrothermal conversion body, comprising: a semiconductor device described above; a jet port provided in correspondence with the electrothermal conversion body serving as the load; a container for accommodating liquid to be supplied to the electrothermal conversion body; and a power supply circuit for supplying a power supply voltage to the semiconductor device.
Other features and advantages of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings.